I created a custom geoprocessing function tool according to this link . . http://help.arcgis.com/en/sdk/10.0/arcobjects_net/conceptualhelp/index.html#/Building_a_custom_geoprocessing_function_tool/00010000049w000000/
I went through the following instructions in order to create a custom tool in a toolbox, but got the error "You do not have the necessary license to execute the selected tool" when attempting to run the custom tool . . .
1) Register the DLL (in the Debug folder) with ESRIRegAsm utility. ESRIRegAsm.exe is located at, ~:\Program Files\Common Files\ArcGIS\bin folder. Use this example . . . Do the following to register the assembly: Desktop—ESRIRegAsm /p:desktop "C:\Test\MyAssembly.dll"
Engine—ESRIRegAsm /p:engine "C:\Test\MyAssembly.dll"
2) Create a custom toolbox in a folder or a geodatabase. ArcGIS for Desktop users have the option to add the tool to a custom toolbox manually in ArcCatalog, ArcMap, or ArcGlobe. Right-click a custom toolbox, choose Add, click Tool, expand my function factory, select my tool, and click OK. The tool is added to the custom toolbox.